1. Field of the Invention
The present invention relates to voltage nonlinear type resistors composed of zinc oxide as a main component.
2. Prior Art Technique
It is widely known that resistors composed mainly of zinc oxide and containing small amounts of additives such as Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, SiO.sub.2, Co.sub.2 O.sub.3, and MnO.sub.2 exhibit excellent voltage-current non-linearity. Such resistors are used for lightning arrestors or the like by utilizing their excellent property
In particular, when the above resistor is used for a lightning arrestors and if excessive current is passed therethrough as a result of a thunderbolt, current is earthed through the voltage non-linear resistor which ordinarily functions as an insulator and which acts as a conductor when a voltage greater than a rated voltage is applied thereto. As a result, accidents due to the thunderbolt Falling can be prevented.
As crystalline phases of the voltage non-linear resistors, bismuth phases of an .alpha. type, a .beta. type, a .gamma. type and a .delta. type as well as a pyrochlore phase exist as intergranular layers in addition to a crystalline phase of zinc oxide. However, depending upon their contents or ratios, a change rate of V.sub.1mA after application of surge current increases or a change rate of a V-I characteristic increases with temperatures. In either case, the characteristic against repeated strikes of thunderbolts may be damaged. Further, when the V.sub.1mA change rate is great like this, there is damage of thermal runaway in the case of a gapless type lightning arrestor, and follow current cannot be interrupted in the case of a gap type lightning arrestor. Further, recent investigations have revealed that depending upon the contents or the ratios of the bismuth places of the .alpha., .beta., .gamma., and .delta. phases or the pyrochlore which exist as the intergranular phase besides the crystalline phase of zinc oxide mentioned above, variations in characteristics such as a voltage non-linearity index or a leakage current ratio becomes greater, and that hygroscopicity of the resistor is deteriorated.